This invention relates to a cylinder member holder, a machining apparatus equipped with the holder and a machining method using the holder. More particularly, the invention relates to art for machining the outer circumferential surface of a cylinder member such as a developing sleeve and photosensitive drum used in an image forming apparatus such as a copier or laser printer, wherein the outer circumferential surface of the cylinder member has a small thickness and a comparatively large length in the axial direction.
Conventionally, lathe machining is carried out when the outer circumferential surface of a cylinder member serving as a workpiece is subjected to cutting or grinding. More specifically, a cylinder member is held immovably in the rotating chuck of a lathe and, while the chuck is being rotated at the same time as the cylinder member by a rotating force from a lathe driving source, a cutting tool such as a cutter held on the apron of the lathe in a freely movable manner is fed toward the surface of the cylinder member to be machined, whereby the cylinder member is subjected to lathe machining. In accordance with such a lathe machining configuration in which the cylinder member is rotated and the cutter is fixed, cutting is carried out by stopping the rotary chuck after a lathe machining operation, removing the machining cylinder member from the chuck, setting a cylinder member in the chuck anew and then rotating the chuck again to perform cutting. This means that an operation for changing the cylinder member is always required when one lathe machining operation ends. A problem that arises is a decline in productivity since extra time is needed to change the cylinder member.
A machining method has been disclosed in the specification of Japanese Patent Application Laid-Open No. 6-328303 in order to improve productivity. According to this disclosed machining method, a cylinder member serving as a workpiece is held in a non-rotatable state by a chuck and a cutting tool is made to rotate about the outer circumferential surface of the cylinder member.
In accordance with this proposal, the operation for changing the cylinder member can proceed without stopping the operation for rotating the tool even after the machining of the outer circumferential surface of the cylinder member is finished. The improvement in productivity is thus achieved.
A cylinder member such as the developing sleeve used in the image forming apparatus described above is incorporated in a developing device in order for an image forming operation to be carried out. The finishing precision of the machined surface of the cylinder member is extremely important. However, when the productivity of the cylinder member is taken into account, the correlation between machining time and finishing precision takes on considerable importance.
Specifically, when the rotational speed of the tool and the feed rate of the tool are raised to improve productivity, chatter on the side of the cylinder member being held fast at both ends is produced, the accuracy of the cut surface declines and this in turn leads to production of defective articles.
As examples of methods for suppressing the occurrence of such chatter, the specifications of Japanese Patent Application Laid-Open Nos. 6-198501 and 6-304803 disclose methods of inserting a vibration damper consisting of a resilient body into the inner diameter portion of a cylinder member and cutting the outer circumferential surface of the cylinder member.
Though suppression of chatter vibration that arises in the cylinder member is performed effectively when such vibration is suppressed in this manner, suppression of chatter that develops on the side of the holder supporting the cylinder member is not always satisfactory. If the outer circumferential surface of the cylinder member is cut under these conditions, a decline in surface accuracy caused by chatter vibration occurs in the vicinity of the supported portions at both ends of the cylinder member held by the holder.
The applicant is aware of the technical documents concerning chatter which happens when cutting cylinder member such as mentioned above, which is titled as "Vibration of machining tools", published by CORONA-sha in 1968, translated by Sakae YONEZU et al., originally published by S. A. TOBIAS.
As shown in FIG. 41, the developing device of an image forming apparatus includes aphotosensitive drum 200 on which a latent image is exposed as image information. A developer G is moved to the drum 200 and is made to adhere thereto via a developing sleeve comprising a cylinder member W. The accuracy at which the developer G can be made to adhere to the photosensitive drum 200 from the developer storage location depends upon the precision to which the surface of the developing sleeve has been machined and the flatness of the sleeve.
Further, the precision to which the photosensitive drum 200 has been machined also has a major influence upon the accuracy (resolution) of the image as a matter of course.
FIG. 42 is a diagram illustrating the units necessary to construct a copier. In FIG. 42, information from an original (not shown) placed on a platen is formed on the photosensitive drum 200 in the form of a latent image, the latent image is then transferred to and fixed on a sheet of copy paper P, and the copy paper P bearing the fixed image is discharged from the apparatus to complete the copying operation.
In the process for fixing the image from the photosensitive drum 200, cylinder members W such as fixing rollers are used. If the precision to which the surfaces of the cylinder members are machined is low, the drum 200 and the photosensitive film develop unevenness. It is known that this has a major adverse influence upon image accuracy.
Good image clarity cannot be obtained unless a high surface accuracy, particularly a high degree of surface roughness precision, is achieved for the cylinder member W, such as the photosensitive drum, developing sleeve and fixing rollers, used in such an image forming apparatus as the copier or laser printer.
In accordance with a conventional method of machining a cylinder member, first a tube material is fabricated by extruding and drawing a metal material such as an aluminum alloy and then applying highly accurate correction of curvature. The tube material is then cut off to a desired length and the outer circumferential surface of the cut length is subjected to cutting by a lathe. However, since the cylinder member W used in the above-described image forming apparatus has a very small wall thickness in comparison with its length, chatter and other vibration easily occurs at cutting and has a deleterious effect upon the precision of surface roughness.
It is known that chatter and vibration which occur when the outer circumferential surface of a cylinder member is cut are produced owing to the relationship among the vibration characteristic of the cylinder member, the vibration characteristic of the holder and the machining frequency. This relationship will now be described.
When use is made of a special-purpose machining apparatus for holding both ends of a cylinder member immovably by left and right holders and cutting the cylinder member at a prescribed feed rate by a cutting tool while the cutting tool is rotated (this apparatus differs from the above-described lathe in that cutting is performed while rotating the cutter about the outer circumferential surface of the cylinder member), it is required that either the left or right holder have a total length through which a unit equipped with a rotary shaft to which the cutter is attached can be passed in order that the entire outer circumferential surface of the cylinder member may be cut. Specifications used in a test were as follows: maximum rotating speed of the unit: 20,000 rpm; actual rotating speed for machining: 9000.about.11,000 rpm; output power: 500W; inner diameter of rotating portion: 35 mm. As for the specifications of the left holder (of open-collet chuck type), through which the unit is passed: total length: 250 mm; maximum outer diameter of portion passed through rotary shaft: 30 mm; material: quenched steel. Since the right holder (of open-collet chuck type) is such that the unit need not be passed through rotary shaft, total length can be shortened. Accordingly, total length of right holder: 80 mm; maximum outer diameter: 30 mm; material: quenched steel.
The cylinder member used in cutting was aluminum alloy (JIS A-6063), had an outer diameter of 16 mm, a wall thickness of 0.8 mm and a total length of 250 mm.
The vibration characteristic, namely the relationship between compliance and frequency, was measured by the impulse excitation method in a state in which only the holders were attached to the cutting apparatus prior to machining and in a state in which the above-mentioned workpiece was supported on the cutting apparatus by the holders. The results are as shown in FIG. 43.
The value of compliance is the reciprocal of dynamic rigidity. The larger the value, the lower the dynamic rigidity and the greater the tendency of excitation.
As shown in FIG. 43, the compliance value gradually increases with an increase in frequency, with the value of compliance (dB) rising sharply at a point A (at which the frequency is about 380 Hz) in both the state in which only the holders were attached to the machining apparatus and the state in which the cylinder member was supported on the apparatus by the holders.
Since the compliance value rises sharply at the same frequency even in the state in which only the left holder is attached, this may be considered the primary characteristic frequency on the side of the special-purpose machining apparatus in the state in which the holders are attached to the apparatus.
Further, since the axial dimension of one holder is required to be of full length so that the unit having the rotary shaft with the attached cutter can be passed through the holder, the characteristic frequency is extremely low.
Though the compliance value decreases in a region of frequencies greater than the frequency at point A, the compliance value for the state in which the cylinder member is supported on the machining apparatus by the holders increases sharply in the vicinity of point B (at which the frequency is about 1000 Hz). This can be regarded as the primary characteristic frequency possessed by the cylinder member W.
Furthermore, the compliance value, which can be regarded as the primary characteristic frequency of the right holder and as the secondary characteristic frequency of the left holder, peaks in the vicinity of point C (at which the frequency is about 1800 Hz).
In a situation where the special-purpose machining apparatus and cylinder member have mutually different characteristic frequencies and a cutting force is applied though the cylinder member by the cutting operation, the cylinder member and special-purpose machining apparatus will resonate if the external force matches these characteristic frequencies. As a result, violent vibration develops even if the cutting force is very small.
FIG. 44 is a diagram drawn to emphasize the state of the cut surface of a workpiece under chatter vibration. It will be appreciated that cutting tracks are left on the surface along the path traversed by the cutting tool.
Accordingly, when the cutting conditions are set, it is necessary that the above-mentioned characteristic frequency and cutting frequency be separated to the utmost degree. The main factors which decide cutting frequency, which is one of the cutting conditions, are tool rpm and number of tools deployed in case of a special-purpose machining apparatus of tool rotating type. Though setting tool rpm high is effective in raising productivity since it contributes to shortening cutting time, vibration (chatter) is produced for the above said reasons when the characteristic frequency is approached.
The table of FIG. 27 illustrates the relationship between absence or presence of chatter vibration and surface roughness of a cylinder member when two cutting tools for coarse and finishing machining were attached to the rotary shaft and the cylinder member was cut while changing tool rpm.
It is obvious from the experimental results that chatter vibration is produced even in a region in which the frequency of the cutting force produced by tool rotation is remote from the above-mentioned characteristic frequency.
In other words, chatter vibration is produced even in a region in which compliance which is less than that at point A in FIG. 43 is increasing.
The specification of Japanese Patent Application Laid-Open No. 64-58453 proposes use of a viscous damper to prevent chatter vibration. According to this proposal, a resilient body such as rubber is inserted into a hollow cylinder member in a case where lathe machining is performed by rotating the hollow cylinder member and holding the tool fast against rotation. By adopting this arrangement, a centrifugal force is produced in the resilient body by rotating the cylinder member, thereby suppressing chatter vibration. With this turning apparatus such as a lathe, the overall length of the holder can be made small, unlike the case with the special-purpose machining apparatus of tool-rotating type. Consequently, the characteristic frequency of the holder can be set high. Further, since the cylinder member is rotated, the rotating speed for cutting is 5000 rpm (83 Hz in case of two cutting tools) at most owing to the problem of dynamic balance. Since even a vibration damper having a comparatively small vibration damping effect is satisfactory, the above-mentioned proposal seems to be practical.
On the other hand, in accordance with the present invention, the above-described special-purpose machining apparatus is used, the cylinder member is held fixed against rotation and cutting is performed while rotating the cutting tool about the outer circumference of the cylinder member. Since the cylinder member subjected to lathe machining does not rotate, a stable dynamic balance is achieved. This means that tool rpm can be set high and that cutting can be performed in a frequency region closer to the characteristic frequency. However, since more comprehensive vibration damping measures are required, it is necessary to use a vibration damping body having a much larger vibration damping effect.
Though an auxiliary mechanism such as pressure means may also be provided to prevent chatter vibration, setting the pressure value of the pressure means and adjusting the mounting position are troublesome operations and therefore this arrangement does not necessarily excel in terms of operability.